Device, method and needle plate for execution of overcasting seams and seams obtained thereby

ABSTRACT

A device ( 1 ) for execution of overcasting seams comprises two needles ( 3, 4 ) carrying respective sewing threads, two support arms ( 6, 9 ) on each of which two lower ( 7, 8 ) and upper ( 10, 11 ) loopers respectively, are mounted. The loopers of each support arm ( 6, 9 ) interact with the loopers of the other arm ( 6, 9 ) and with the needles ( 3, 4 ) to form a first and a second overcasting seam portions superposed on each other. The invention also relates to a method to be put into practice by such a device ( 1 ), a needle plate to be installed on such a device ( 1 ) and a seam consequently obtainable therewith.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a device for execution of overcasting seams comprising at least one main needle carrying a respective sewing thread, movable with a rectilinear reciprocating motion through a needle plate along which a workpiece runs following a sewing direction; at least one lower support arm alternately oscillatable around a rotation axis substantially parallel to the sewing direction of said workpiece; at least one main lower looper mounted on said lower arm in such a manner that said main lower looper is alternately movable according to an arched trajectory in a direction transverse to the sewing direction; at least one upper arm oscillatable around an oscillation axis substantially parallel to the sewing direction of said workpiece; at least one main upper looper mounted on said upper arm in such a manner that said main upper looper is alternately movable according to an arched trajectory extending astride a side edge of said workpiece in a direction transverse to the sewing direction of the workpiece itself, said main upper looper interacting with said main lower looper and main needle to cause formation of an overcasting seam portion on said workpiece.

[0003] The invention also relates to a method of executing overcasting seams comprising the steps of carrying out a main seam portion of a predetermined sewing pitch on a workpiece according to its sewing direction; carrying out an auxiliary seam portion superposed on the main seam portion and of a predetermined sewing pitch on said workpiece.

[0004] It is also an object of the invention to provide a needle plate for execution of overcasting seams comprising a main slot having an end portion arranged to be passed through by a main needle carrying a respective main sewing thread to make a main seam portion on a cloth workpiece moving forward on the needle plate in a sewing direction substantially parallel to a primary side of the needle plate; an auxiliary slot having an end portion arranged to be passed through by an auxiliary needle carrying a respective auxiliary sewing thread to make an auxiliary seam portion on said cloth workpiece being fed; at least one main tab formed at the primary side of said needle plate and partly bounding at least one of said slots.

[0005] It is a further object of the invention to provide an overcasting seam comprising a main overcasting portion having a series of sewing stitches of a predetermined pitch; and an auxiliary overcasting portion superposed on said main seam and having a series of sewing stitches of a predetermined pitch.

[0006] 2. Prior Art

[0007] It is known that in manufacturing garments such as socks, stockings, tights and knitwear in general, the articles of manufacture are submitted to assembling operations to unite the different parts of the fabric, and/or to aesthetic-finishing operations at the edges of the obtained article of clothing. Usually both assembling and finishing are carried out by the so-called trimming-sewing machines set to carry out an overcasting seam.

[0008] For execution of an overcasting seam the trimming-sewing machines are equipped with a sewing device having a needle carrying a sewing thread provided with a reciprocating movement in a rectilinear direction. The needle is set to pass through a needle plate on which the workpiece is designed to move forward.

[0009] Such a device further comprises a lower arm and an upper arm alternately oscillatable around a rotation axis parallel to the feed direction of the workpiece. A lower looper carrying a respective interlinking thread is mounted in engagement with the lower arm so that the lower looper is alternately movable following an arched trajectory in a direction substantially perpendicular to the feed direction of the workpiece. Mounted on the upper arm is a respective upper looper possibly provided with a respective thread, alternately movable according to an arched trajectory substantially perpendicular to the feed direction of the workpiece. Cooperation between the needle, lower looper and upper looper gives rise to the desired overcasting seam.

[0010] In order to make the finished product stronger and aesthetically improved, along the junction edges of the fabric parts to be joined, several superposed overcasting seams are generally carried out.

[0011] In automatic working processes, for carrying out the above mentioned seams apparatus consisting of two sewing machines are often employed. A first machine, usually consisting of a trimming-sewing machine carries out a first overcasting seam and a second machine disposed in series with the first machine, carries out a second overcasting seam superposed on the first seam. The first and second machines are connected with each other by a transferring guide by which each cloth workpiece previously worked in the first machine, is sent to the second one.

[0012] More particularly, at the first machine, the fabric parts forming the workpiece are submitted to a cutting operation to determine a perfect mating of the junction edges of said parts. Simultaneously with the cutting operation and immediately downstream of the region where this operation is carried out, the first machine is set to execute an overcasting seam causing assembling of the fabric parts forming the workpiece.

[0013] The cloth workpiece is then moved on along the transferring guide to reach the second machine, at which a further overcasting seam is carried out in superposition with the seam executed by the first machine.

[0014] The foregoing being stated, the Applicant has found that known apparatus and techniques for executing junctions between the fabric parts in the manufacture of cloth articles by means of superposed overcasting seams are not exempt from some drawbacks and can be improved from different points of view mainly as regards the quality of the obtained seams as well as the positioning regularity and accuracy between the stitches of the different seams. In fact, generally the interposition between stitches of the different seams is inhomogeneous because the workpiece that has already been submitted to a first overcasting operation is transferred from the first machine or trimming-sewing machine to the second machine where another overcasting seam is carried out. During this transferring the workpiece above all if it is made of an elastically yielding material, is submitted to elongation and/or shortening, its starting structure being deformed. The structural deformations to which each workpiece is submitted during transferring cause inaccuracy in the arrangement of the stitches in the executed seams, consequently giving a quality level which is not completely satisfactory to the obtained product.

[0015] In addition, being the second seam necessarily carried out in the absence of trimming of the edges of the fabric parts, its correct execution is greatly conditioned by an imperfect alignment of the workpiece with the needles and the other sewing members.

[0016] Consequently, each minimum transverse shifting or deviation of the workpiece affects a correct execution of the seam.

[0017] It should be also considered that these apparatus are not adapted to carry out seams of the curvilinear type that therefore must be executed manually by an operator.

[0018] It will be also understood that these apparatus are greatly expensive because they contemplate the presence of two sewing machines and a series of devices adapted to make said machines automatic causing them to operate in synchronism with the workpiece-feeding system.

SUMMARY OF THE INVENTION

[0019] The present invention aims at solving the problems encountered in the known art by proposing a device and method for execution of overcasting seams ensuring superposed seams of high quality and accuracy as regards seams of both the rectilinear and the curvilinear type.

[0020] It is a further aim of the present invention to propose a needle plate to be combined with said device to further improve the quality of the superposed overcasting seam portions, particularly in terms of accuracy of same.

[0021] The foregoing and further aims that will become more apparent in the course of the following description are substantially achieved by a device for execution of overcasting seams comprising at least one auxiliary needle carrying a respective auxiliary sewing thread, movable with a rectilinear reciprocating motion through said needle plate; at least one auxiliary lower looper mounted on said lower arm in such a manner that said auxiliary lower looper is alternately movable following an arched trajectory in a direction transverse to the sewing direction; at least one auxiliary upper looper mounted on said upper arm in such a manner that said auxiliary upper looper is movable according to an arched trajectory extending astride a side edge of said workpiece in a direction transverse to said sewing direction, said auxiliary upper looper cooperating with said auxiliary lower looper and said auxiliary needle to give rise to formation of an auxiliary overcasting seam portion on said workpiece.

[0022] Said aims are further achieved by a method of executing overcasting seams wherein each stitch of the main seam portion is formed simultaneously with the formation of a corresponding stitch of the auxiliary seam portion, said main seam portion being executed with the same sewing pitch as the sewing pitch of said auxiliary seam portion, as well as by a needle plate for execution of overcasting seams having at least one auxiliary tab formed along said primary side of the needle plate substantially in alignment with the main tab, said auxiliary tab bounding said auxiliary slot at least partly.

[0023] The above-mentioned aims are also achieved by an overcasting seam wherein the pitch of the stitches of said main portion is the same as the pitch of the stitches of said auxiliary portion.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Further features and advantages will be best understood from the detailed description of a preferred but not exclusive embodiment of a device, a method and a needle plate for execution of overcasting seams and of a seam obtained with use of the same, in accordance with the present invention. This description will be set out hereinafter with reference to the accompanying drawings given by way of non-limiting example, in which:

[0025]FIG. 1 is a perspective view of a device and a needle plate for execution of overcasting seams in accordance with the present invention;

[0026]FIG. 2 is a top view of the needle plate shown in FIG. 1;

[0027]FIG. 3 is a section of the needle plate taken along line III-III in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0028] With reference to the drawings, a device for execution of overcasting seams in accordance with the present invention has been generally identified by reference numeral 1.

[0029] Such a device 1 can be advantageously installed on machines set to carry out overcasting seams.

[0030] As viewed from the accompanying figure, device 1 comprises at least one needle bar 2 alternately movable according to a substantially rectilinear trajectory. Fastened to the needle bar 2 by appropriate locking means is at least one main needle 3 and at least one auxiliary needle 4 both oriented downwardly and carrying a main sewing thread and an auxiliary sewing thread (not shown), respectively. The main needle 3 and auxiliary needle 4 move therefore integrally with the needle bar 2 in a rectilinear reciprocating motion through a needle plate 5 located below the needles and along which a workpiece runs in a sewing direction denoted by arrow “A” in the accompanying figure.

[0031] As shown in FIG. 1, the main 3 and auxiliary 4 needles lie in a plane transverse to the sewing direction “A” of the workpiece. Preferably needles 3, 4 lie in a plane substantially oblique to the sewing direction “A” defining an inclination angle “α” therewith, of an opening included between 1° and 70°, measurable in the lying plane of the needle plate 5.

[0032] Device 1 further comprises at least one lower arm 6 disposed laterally of the needle plate 5 and rigidly connected, by an end 6 a thereof, with a lower drive shaft (not shown as known by itself) alternately oscillatable around a longitudinal axis “X” thereof.

[0033] Locked to the other end 6 b of the lower arm 6, opposite to end 6 a, is at least one main lower looper 7 and at least one auxiliary lower looper 8.

[0034] The lower loopers 7, 8 are alternately movable simultaneously with each other, following an arched trajectory in a direction perpendicular to the sewing direction “A” of the workpiece. The movement direction of each lower looper 7, 8 intersects the movement direction of the respective needle 3, 4 with which the respective sewing stitch is formed.

[0035] The main lower looper 7 and auxiliary lower looper 8 carry a main interlinking thread and an auxiliary interlinking thread (not shown), respectively, which are intended to be interlinked with the main sewing thread and the auxiliary sewing thread carried by the respective needles 3, 4 to generate, as better clarified in the following, a main seam portion and an auxiliary seam portion superposed on the main seam portion, respectively.

[0036] Opposite to the lower support arm 6 and laterally of the needle plate 5, the device further involves an upper support arm 9 connected by an end 9 a thereof to a kinematic drive mechanism (not shown as known per se) and provided, at the other end 9 b thereof opposite to end 9 a, with at least one main upper looper 10 and at least one auxiliary upper looper 11. During operation of device 1 the kinematic drive mechanism sets the upper arm 9 in motion so that both the upper loopers 10, 11 follow a movement trajectory extending astride an edge of the workpiece in a direction substantially transverse to the sewing direction “A” of the workpiece itself.

[0037] The main upper looper 10 and/or the auxiliary upper looper 11 may possibly carry respective interlinking threads designed to form, together with the interlinking threads of the lower loopers 7, 8 and of needles 3, 4, the stitches of the main and auxiliary seam portions to be executed.

[0038] As shown in the accompanying figures, the needle plate 5 has a substantially plate-like structure made up of a first portion 12 which is substantially rectangular and a second portion 13 which is substantially trapezoidal and of one piece construction with the first portion 12 from which it laterally projects in cantilevered fashion, on the opposite side with respect to a primary side 17 substantially parallel to the sewing direction “A” and defined along the perimetral extension of the needle plate 5.

[0039] To enable fastening to a machine for execution of overcasting seams, the needle plate 5 is provided with fastening holes 14 a, 14 b to be engaged by respective screw-threaded elements not shown.

[0040] The needle plate 5 can be provided with one or more central openings 15 through which one or more feed dogs operate which are part of a feeding device (not shown as known per se) acting on the workpiece to cause moving forward of same in the above-mentioned sewing direction “A”.

[0041] In more detail, in the embodiment shown, central openings 15 disposed in mutual side by side relationship are provided and they have an elongated configuration in a direction parallel to the sewing direction; also provided are two auxiliary openings 16 interposed in parallel between the central openings 15 and the primary side 17.

[0042] The needle plate 5 further has a main slot 21 and an auxiliary slot 22 formed close to the primary side 17. The main slot 21 and auxiliary slot 22 are arranged to be operatively passed through by the main needle 3 and auxiliary needle 4 of the above described device, respectively.

[0043] As shown in FIGS. 1 and 2, each slot 21, 22 has a substantially L-shaped conformation defined by a first stretch 21 a, 22 a substantially parallel to the primary side 17 of the needle plate 5 and a second stretch 21 b, 22 b substantially perpendicular to the first stretch 22 a and extending between the first stretch and the primary side 17 so as to break continuity thereof.

[0044] The first stretch 21 a, 22 a of each slot 21, 22 has an end portion 21 c, 22 c opposite to the respective stretch 21 b, 22 b and designed to be passed through by the respective needle 3, 4.

[0045] In accordance with the arrangement of needles 3, 4 of device 1, the end portions 21 c, 22 c of slots 21, 22 are disposed mutually in side by side relationship in a direction transverse to the sewing direction “A” and to the primary side 17 of the needle plate 5 in such a manner that the distance between the end portion 21 c of the main slot 21 and the primary side 17 is different from the distance between said primary side and the end portion 22 c of the auxiliary slot 22.

[0046] It should be noted however that, depending on requirements, slots 21, 22 can also be formed so as to be mutually aligned along a direction substantially parallel to the primary side 17.

[0047] The needle plate 5 further involves the presence of a main tab 23 and an auxiliary tab 24 formed at the primary side 17 of the needle plate 5 itself and at least partly bounding the respective main 21 and auxiliary 22 slots; said tabs each have an end edge 23 a, 24 a spaced apart from the end portion 21 c, 22 c of the corresponding slot 21, 22 and facing away from said slots.

[0048] As viewed from FIGS. 1 and 2, tabs 23, 24 are substantially aligned along the primary side 17 of the needle plate 5. In other words, the end edge 23 a, 24 a of each tab 23, 24 is preferentially in alignment with the end edge 23 a, 24 a of the other tab 23, 24 along one and the same direction parallel to the sewing direction “A”.

[0049] However, depending on requirements, it is also possible to form tabs 23, 24 on the needle plate 5 which have end edges 23 a, 24 a offset with respect to a direction parallel to the sewing direction “A”.

[0050] The border height of each seam portion is determined by the distance measurable between the end edge 23 a, 24 a of each tab 23, 24 and the end portion 21 c, 22 c of the corresponding slot 21, 22.

[0051] Therefore, by intervening on the position of slots 21, 22 and/or the end edges 23 a, 24 a of tabs 23, 24, superposed seam portions can be obtained in which the overlying seam portion completely covers the underlying seam portion in width, or in which the overlying seam portion only partly overlaps the underlying seam portion.

[0052] In addition it is preferably provided that the end edge 23 a, 24 a of each tab 23, 24 should have at least one tail portion 23 b, 24 b terminating at the second stretch 21 b, 22 b, of the respective slot 21, 22 and disposed obliquely of the primary side 17 of the needle plate 5 with an inclination converging towards the respective slot 21, 22 away from the end portion 21 c, 22 c of said slot.

[0053] The inclination of the tail portions 23 b, 24 b of the respective tabs 23, 24 affects the course according to which the formed sewing stitches disengage the tabs themselves and move forward on the needle plate 5.

[0054] In order to achieve a constant and almost similar course between the main seam portion and the auxiliary seam portion being formed, the tail portion 23 b of the main tab 23 is preferably oriented substantially parallel to the tail portion 24 b of the auxiliary tab 24.

[0055] However, in order to achieve typical and/or elegant seams, the tail portions 23 b, 24 b may have an inclination degree different from each other, consequently giving rise to different courses in the forward or feed movement of the different seam portions being executed. The needle plate 5 further comprises a lead-in portion 25 interposed between the main tab 23 and the auxiliary tab 24 to convey the main seam portion moving forward along the main tab 23, towards and upon the auxiliary tab 24. In more detail, the lead-in portion 25 has a first end 25 a disposed at a lower position relative to the main tab 23 and a second end 25 b, opposite to the first end 25 a, of one piece construction with the auxiliary tab 24 so as to define an inclined plane extending from bottom to top away from the main tab 23, until it becomes coincident with the upper surface of the auxiliary tab 24. In other words, the lead-in portion is defined by a chamfer extending from the auxiliary tab 24 downwardly towards the main tab 23 to accompany the main seam portion over the needle plate 5 before the main seam portion itself reaches the auxiliary tab 24 where the auxiliary seam portion is made.

[0056] The needle plate 5 further comprises an end portion 26 provided with a further chamfer 27 to cause the superposed seam portions from the auxiliary tab 24 to move forward.

[0057] Operation of the device 1 provided with a needle plate in accordance with the present invention is now described.

[0058] By appropriate feed means the cloth workpiece is guided onto the needle plate 5 of the trimming-sewing machine, at which a trimming operation is carried out, immediately upstream of needles 3, 4, on the edge of the workpiece to be sewn.

[0059] When the workpiece is close to needles 3, 4 and to the lower 7, 8 and upper 10, 11 loopers, the fabric parts are joined together by two superposed overcasting seam portions. A main seam portion is formed by cooperation between the main needle 3, the main lower looper 7 and the main upper looper 10 around the main tab 21, and an auxiliary seam portion is formed by interaction between the auxiliary needle 4, the auxiliary lower looper 8 and the auxiliary upper looper 11 around the auxiliary tab 22.

[0060] It should be noted that the stroke of the main lower looper 7 has the same length as the stroke of the auxiliary lower looper 8 since both loopers are fastened to the same lower support arm 6.

[0061] However, in accordance with the present invention it is preferable for the auxiliary seam portion to have a border height, i.e. a distance between the sewing stitch and the respective edge of the materials to be joined together, greater than the border height of the main seam portion. This difference between the border heights of the seam portions is determined by the offset arrangement of needles 3, 4 relative to the sewing direction “A” which needles act through the respective offset slots 21, 22 of the needle plate 5. In other words, since it lies in a plane transverse to the sewing direction “A”, the main needle 3 forms a stitch on the workpiece, close to the main slot 21, which is closer to the junction edge of the fabric parts to be joined than the stitch formed by the auxiliary needle 4 at the auxiliary slot 22.

[0062] In order to ensure a correct interaction with needles 3, 4, the lower loopers 7, 8 and upper loopers 10, 11 may be provided to be mutually offset in the respective movement directions, by an extent corresponding to the distance between centers measurable between the needles 3, 4 in a direction normal to the sewing direction “A”.

[0063] When working is over, the main and auxiliary seam portions of the obtained overcasting seam each have a series of sewing stitches having a predetermined pitch. The pitch of the sewing stitches of the main portion is the same as the pitch of the sewing stitches of the auxiliary portion and each sewing stitch of the auxiliary portion is spaced a predetermined distance from the preceding stitch of the main portion, which distance may for example correspond to half the sewing pitch.

[0064] The present invention solves the problems found in the known art and achieves the intended purposes.

[0065] First of all the device being the object of the invention enables superposed overcasting seams to be obtained which have a high accuracy degree in the interposition between the stitches of said seams. In more detail, such a device enables each sewing stitch to be surely spaced from the preceding and following stitches by a desired and planned distance. In other words, if for example it has been provided for the auxiliary sewing stitches to be coincident with the middle point measurable between the main sewing stitches, such a configuration will not be submitted to any variation during the whole working and will give, as a final result, a precise and homogeneous double overcasting seam. This condition is made possible by the vicinity between the elements adapted to make the main seam portion or first seam and the elements adapted to make the auxiliary seam portion or second seam. In fact, the workpiece is not submitted to long displacements any more, along guides or other similar conveying means bringing it from the region where the first seam takes place to the region where the second seam is executed, but it is directly treated in a single region involving execution of both seams simultaneously.

[0066] The accuracy in interlinking the stitches of the first seam with those of the second seam enables achievement of seams of high quality even on curvilinear stretches or edges of the material, carrying out feeding of the workpiece either manually or automatically.

[0067] It should be also considered that, due to the elimination of transferring guides to direct the workpiece, all problems connected with accuracy in guiding the workpiece in the second machine are eliminated as well, and therefore seams can be made at much higher speeds without impairing accuracy in sewing.

[0068] In addition, the production costs are greatly reduced as compared with the known art because the article of manufacture that was previously made using two sewing machines, can be now obtained by use of a single machine. Use of a single sewing machine has also made it possible to eliminate the transferring guides necessary to move the workpiece from a machine to the other and consequently to eliminate the complicated synchronism systems currently adapted in the known art to organize the operations of the two machines and displacement of the article of manufacture.

[0069] Contrary to the needle plates generally employed, use of a needle plate 5 in accordance with the invention in association with the above-mentioned device 1 enables the quality of the obtained seams to be greatly improved because device 1 is adapted to give the best in terms of functional performance. In more detail, the presence of a tab 23, 24 for each slot 21, 22 causes each of the seam portions to be disengaged from the needle plate in an optimal manner, without causing inhomogeneity in the stitch distribution and/or creasing or puckering of the material.

[0070] It should be noted in this connection that use of needle plates made in accordance with the known art in which both seam portions would be obtained around a single tab, makes it practically impossible to give the tab geometrical features adapted, on the one hand, to ensure an optimal running of the first seam portion along the tab itself and, on the other hand, to make a second seam portion sufficiently loose to enable subsequent extension of the sewn materials in a single plane without the article of manufacture being interstitched with an undesired constraining effect in the second seam portion itself.

[0071] In addition, due to the presence of two distinct tabs, the width of the two obtained seam portions can be managed separately and independently of each other. Thus it is possible, depending on the requirements encountered each time, to superpose on the first seam portion, a second seam portion of greater or smaller width or border height than the first seam portion or of same width or border height as said first seam portion. 

What is claimed is:
 1. A device for execution of overcasting seams comprising: at least one main needle (3) carrying a respective sewing thread, movable with a rectilinear reciprocating motion through a needle plate (5) along which a workpiece runs following a sewing direction (“A”) at least one lower support arm (6) alternately oscillatable around a rotation axis (“X”) substantially parallel to the sewing direction (“A”) of said workpiece; at least one main lower looper (7) mounted on said lower arm (6) in such a manner that said main lower looper (7) is alternately movable according to an arched trajectory in a direction transverse to the sewing direction (“A”); at least one upper arm (9) oscillatable around an oscillation axis substantially parallel to the sewing direction (“A”) of said workpiece; at least one main upper looper (10) mounted on said upper arm (9) in such a manner that said main upper looper (10) is alternately movable according to an arched trajectory extending astride a side edge of said workpiece in a direction transverse to the sewing direction (“A”) of the workpiece itself, said main upper looper (10) interacting with said main lower looper (7) and main needle (3) to cause formation of an overcasting seam portion on said workpiece, characterized in that it further comprises: at least one auxiliary needle (4) carrying a respective auxiliary sewing thread, movable with a rectilinear reciprocating motion through said needle plate (5); at least one auxiliary lower looper (8) mounted on said lower arm (6) in such a manner that said auxiliary lower looper (8) is alternately movable following an arched trajectory in a direction transverse to the sewing direction (“A”); at least one auxiliary upper looper (11) mounted on said upper arm (9) in such a manner that said auxiliary upper looper (11) is movable according to an arched trajectory extending astride a side edge of said workpiece in a direction transverse to said sewing direction (“A”), said auxiliary upper looper (11) cooperating with said auxiliary lower looper (8) and said auxiliary needle (4) to give rise to formation of an auxiliary overcasting seam portion on said workpiece.
 2. The device as claimed in claim 1, wherein said auxiliary needle (4) and main needle (3) are engaged on one and the same needle bar (2).
 3. The device as claimed in claim 1, wherein said main needle (3) and auxiliary needle (4) lie in a plane transverse to the sewing direction (“A”).
 4. The device as claimed in claim 3, wherein the lying plane of said needles (3, 4) is oblique to the sewing direction (“A”).
 5. The device as claimed in claim 4, wherein the lying plane of said needles (3, 4) is inclined relative to the sewing direction of the workpiece, by an angle included between 1° and 70° measurable on the lying plane of the needle plate (5).
 6. The device as claimed in claim 1, wherein said needle plate (5) has: a main slot (21) having an end portion (21 c) arranged to be passed through by the main needle (3); an auxiliary slot (22) having an end portion (22 c) arranged to be passed through by the auxiliary needle (4); at least one main tab (23) formed at the primary side (17) of said needle plate (5) and partly bounding at least one of said slots (21, 22); at least one auxiliary tab (24) formed along said primary side (17) of the needle plate (5) substantially in alignment with the main tab (23), said auxiliary tab (24) bounding said auxiliary slot (22) at least partly.
 7. The device as claimed in claim 6, wherein each tab (23, 24) has an end edge (23 a, 24 a) spaced apart from the end portion (21 c, 22 c) of the respective slot (21, 22) and facing away from said slot, the end edges (23 a, 24 a) of said tabs (23, 24) being mutually aligned along the same direction.
 8. The device as claimed in claim 7, wherein said main slot (21) and auxiliary slot (22) are disposed in mutual side by side relationship in a direction transverse to said sewing direction (“A”).
 9. The device as claimed in claim 8, wherein the end portion (21 c) of the main slot (21) is spaced apart from the end edge (23 a) of said main tab (23) by a lower distance than the distance existing between the end edge (24 a) of said auxiliary tab (24) and the end portion (22 c) of the auxiliary slot (22).
 10. The device as claimed in claim 9, wherein the end portion (21 c) of the main slot (21) is spaced apart from the end edge (23 a) of said main tab (23) by a distance greater than the distance existing between the end edge (24 a) of said auxiliary tab (24) and the end portion (22 c) of the auxiliary slot (22).
 11. The device as claimed in claim 10, wherein the end portion (21 c) of the main slot (21) is spaced apart from the end edge (23 a) of said main tab (23) by the same distance as the distance existing between the end edge (24 a) of said auxiliary tab (24) and the end portion (22 c) of the auxiliary slot (22).
 12. The device as claimed in claim 6, wherein said slots (21, 22) are mutually aligned along a direction substantially parallel to said primary side (17).
 13. The device as claimed in claim 7, wherein the end edge (23 a, 24 a) of each tab (23, 24) has at least one tail portion (23 b, 24 b) oriented obliquely of said primary side (17) with an inclination converging towards the respective slot (21, 22), away from the end portion (21 c, 22 c) of the slot itself.
 14. The device as claimed in claim 6, wherein said needle plate (5) further comprises a lead-in portion (25) interposed between said main tab (23) and said auxiliary tab (24) to convey the main seam portion moving forward along the main tab (23), towards and upon the auxiliary tab (24).
 15. The device as claimed in claim 14, wherein said lead-in portion (25) has a first end (25 a) disposed below said main tab (23) and a second end (25 b) opposite to the first end (25 a), of one piece construction with said auxiliary tab (24).
 16. A method of executing overcasting seams comprising the steps of: carrying out a main seam portion of a predetermined sewing pitch on a workpiece according to its sewing direction; carrying out an auxiliary seam portion superposed on the main seam portion and of a predetermined sewing pitch, on said workpiece, characterized in that each stitch of the main seam portion is formed simultaneously with the formation of a corresponding stitch of the auxiliary seam portion, said main seam portion being executed with the same sewing pitch as the sewing pitch of said auxiliary seam portion.
 17. The method as claimed in claim 16, wherein each stitch of said auxiliary seam portion is formed at a predetermined offsetting distance from the corresponding stitch of the main seam portion preceding it, the offsetting distance being the same for all stitches of said seam.
 18. The method as claimed in claim 17, wherein the offsetting distance between each sewing stitch of the auxiliary seam portion and the corresponding sewing stitch of the main seam portion corresponds to half the sewing pitch.
 19. A needle plate for execution of overcasting seams provided with: a main slot (21) having an end portion (21 c) arranged to be passed through by a main needle (3) carrying a respective main sewing thread to make a main seam portion on a cloth workpiece moving forward on the needle plate (5) in a sewing direction (“A”) substantially parallel to a primary side (17) of the needle plate (5); an auxiliary slot (22) having an end portion (22 c) arranged to be passed through by an auxiliary needle (4) carrying a respective auxiliary sewing thread to make an auxiliary seam portion on said cloth workpiece being fed; at least one main tab (23) formed at the primary side (17) of said needle plate (5) and partly bounding at least one of said slots (21, 22), characterized in that it further has at least one auxiliary tab (24) formed along said primary side (17) of the needle plate (5) substantially in alignment with the main tab (23), said auxiliary tab (24) bounding said auxiliary slot (22) at least partly.
 20. The needle plate as claimed in claim 19, wherein each tab (23, 24) has an end edge (23 a, 24 a) spaced apart from the end portion (21 c, 22 c) of the respective slot (21, 22) and facing away from said slot, the end edges (23 a, 24 a) of said tabs (23, 24) being mutually aligned along the same direction.
 21. The needle plate as claimed in claim 20, wherein said main slot (21) and auxiliary slot (22) are disposed in mutual side by side relationship in a direction transverse to said sewing direction (“A”).
 22. The needle plate as claimed in claim 21, wherein the end portion (21 c) of the main slot (21) is spaced apart from the end edge (23 a) of said main tab (23) by a lower distance than the distance existing between the end edge (24 a) of said auxiliary tab (24) and the end portion (22 c) of the auxiliary slot (22).
 23. The needle plate as claimed in claim 22, wherein the end portion (21 c) of the main slot (21) is spaced apart from the end edge (23 a) of said main tab (23) by a distance greater than the distance existing between the end edge (24 a) of said auxiliary tab (24) and the end portion (22 c) of the auxiliary slot (22).
 24. The needle plate as claimed in claim 23, wherein the end portion (21 c) of the main slot (21) is spaced apart from the end edge (23 a) of said main tab (23) by the same distance as the distance existing between the end edge (24 a) of said auxiliary tab (24) and the end portion (22 c) of the auxiliary slot (22).
 25. The needle plate as claimed in claim 19, wherein said slots (21, 22) are mutually aligned along a direction substantially parallel to said primary side (17).
 26. The needle plate as claimed in claim 20, wherein the end edge (23 a, 24 a) of each tab (23, 24) has at least one tail portion (23 b, 24 b) oriented obliquely of said primary side (17) with an inclination converging towards the respective slot (21, 22), away from the end portion (21 c, 22 c) of said slot.
 27. The needle plate as claimed in claim 19, further comprising a lead-in portion (25) interposed between said main tab (23) and said auxiliary tab (24) to convey the main seam portion moving forward along the main tab (23), towards and upon the auxiliary tab (24).
 28. The needle plate as claimed in claim 27, wherein said lead-in portion (25) has a first end (25 a) disposed below said main tab (23) and a second end (25 b) opposite to the first end (25 a), of one piece construction with said auxiliary tab (24).
 29. An overcasting seam comprising: a main overcasting portion having a series of sewing stitches of a predetermined pitch; an auxiliary overcasting portion superposed on said main seam and having a series of sewing stitches of a predetermined pitch, characterized in that the pitch of the stitches of said main portion is the same as the pitch of the stitches of said auxiliary portion.
 30. The seam as claimed in claim 29, wherein each sewing stitch of said auxiliary seam portion is spaced apart from the preceding stitch of the main seam portion by a distance corresponding to half the sewing pitch. 